In a typical optical video recorder, a laser beam is focused onto the surface of an optically-recordable coating of a recording disc and pulsed corresponding to a pattern of information. The laser transfers enough energy to the optically-recordable coating to burn, evaporate, melt or otherwise create a pattern of discrete optically-recordable sites which are sometimes called "pits". The information is generally represented by the sizes of and spacings between the pits.
The optically-recordable coating may be a metal coating on the order of 30 to 50 nm in thickness on a highly polished glass substrate, in which case it is necessary to create a greater relief before the recording can be physically replicated. To do this, a photo-resist coating may be applied, irradiated through the pits and developed. A metal vapor coating is applied to make the developed surface conductive, and this is plated to produce a metal, usually nickel, stamper from which a large number of replicates can be made. See U.S. Pat. No. 3,954,469.
Another master optical recording disc is shown in European patent application No. 78300865.9 filed Dec. 19, 1978 which is based on U.S. patent application Ser. No. 862,069 filed Dec. 19, 1977. A support such as glass carries a thin layer of amorphous material comprising a thermoplastic resin binder and a substance capable of absorbing energy from a pulse-encoded laser beam, thereby recording information in the form of depressions or holes surrounded by sharply defined ridges. The absorbing substance may be a dye chosen for its high absorptivity at the wavelength of the recording beam. Although each of the examples uses a glass plate as the support, it is said that the support may be a resin film such as polyethylene terephthalate which has sufficiently high melting point to avoid deformation during recording. The energy-absorbing layer is solvent-coated such as by bar-coating, spray-coating or whirl-coating. The thickness of the layer should be less than 0.45 micrometer, because thicker layers have heat-dissipation and flow properties that render them incapable of forming the desired depressions or holes. The energy-absorbing layer should have a glass transition temperature significantly higher than room temperature to be able to retain the thermal deformation pattern after recording. It is said that the boundaries of the depressions or holes are sufficiently sharp to provide a signal-to-noise ratio on playback of 40 decibels or greater.
The above-described optical recording disks have direct-read-after-write (DRAW) capability which permits adjustments to be made while the master optical disc is being recorded. DRAW capability also eliminates the time and expense of subsequent processes to make the recording readable.
In spite of the advantages of DRAW capability, it is believed that all or nearly all master optical recording discs in current production do not have this capability. Instead a photo-resist coating is applied to a glass substrate, usually after first applying an adhesion-promoting layer. After exposure a metal stamper is made from the developed coating. A plastic replicate made from the stamper must be tested before it is known whether the stamper is of adequate quality. Because it is difficult to fix the recording conditions in the absence of DRAW capability, the rejection rate of these stampers is currently rather high. Even if there were no rejections, the process would be quite expensive because of the cost of polishing the glass substrate to the desired smoothness and flatness.
Plastic replicates made from the stamper tend to expand and contract with changes in temperature, thus making it difficult to locate with precision specific recorded information. Plastic replicates also may warp to such an extent that it may be impossible to maintain focus during readback at the high speeds at which video discs are commonly rotated.